1. Field of the Invention
This invention relates to a method of manufacturing semiconductor devices, and more particularly to an improved method of forming electrodes and interconnections of semiconductor devices.
2. Description of the Prior Art
In general, the resistance values of electrodes and interconnections of semiconductor devices, particularly of MOS (metal-oxide-semiconductor) type transistors have significant effects upon their operating speeds, and also upon the degree of freedom in their design. Thus, these resistance values are required to be minimized. For this reason, various techniques have been disclosed in which a refractory metal (having a high melting point and a relatively low specific resistance) is used to form the gate electrodes and interconnections. When a refractory metal is deposited on a polycrystalline silicon film in a MOS-type transistor, the work function of the gate at the MOS capacitor portion is substantially the same as that in the case of the conventional polycrystalline silicon gate. In other words, the work function of the gate is changed when the refractory metal film alone is used as a single-layer structure gate. Thus, the structure that the refractory metal film is laminated on the polycrystalline silicon film is advantageous more than the single-layer structure of the refractory metal.
However, the process of manufacturing MOS-type semiconductor devices generally includes several high temperature processes. Specifically, the manufacturing process requires a post oxidization process (later described in detail) of about 700.degree. C. to 800.degree. C. and an inter-layer film leveling process of about 700.degree. C. to 850.degree. C., for example. These temperatures are well over the temperature at which the refractory metal combines with silicon in a polycrystalline silicon film. (e.g. 550.degree. C. to 600.degree. C. for tungsten). To avoid such chemical reaction between the polycrystalline silicon film and the refractory metal film, a metal nitride film or a metal carbide film is interposed therebetween to prevent conducting and refractory metal films from the reaction.
The post oxidization process is performed to recover defects and damage to the substrate, which are induced during etching or ion implantation. The post oxidization process is also intended to enhance the gate characteristics of a MOS capacitor and a MOS-type transistor. However, the refractory metal film is easily oxidized in the post oxidization process. Particularly in an atmosphere of oxygen at about 900.degree. C. (ordinary oxidization condition for a polycrystalline silicon film), or in an atmosphere of O.sub.2 -H.sub.2 O at about 800.degree. C., the refractory metal film is significantly oxidized. As a result, the resistance values of the gate electrode and interconnections, which are made of refractory metal, inevitably increase.
To prevent such an increase of the resistance values of the electrodes and interconnections, it is necessary to oxidize only the silicon film, while the refractory film is not oxidized. This can be achieved if the oxidization process is performed in an atmosphere of hydrogen containing steam of a few percent at about 900.degree. C. However, such oxidization process requires hydrogen of high concentration at a high temperature. This process is sometimes accompanied with a danger of explosion. Moreover, the oxidization rate of the polycrystalline silicon film is very slow in the process. As a result, the necessary oxidization of the polycrystalline silicon film inevitably requires a long-time processing. This has been a restriction in manufacturing MOS-type transistors.